Ungrounded power distribution systems are widely used, especially at medium voltage levels, e.g., less than 50 kV. Compared with the grounded distribution systems, the ungrounded systems do not have neural wires to connect with the ground, and they are connected to ground through phase-to-ground capacitances of power lines. When a single-phase-to-ground fault occurs, the fault currents of ungrounded systems are less than normal load currents, thus the system can continue to operate until the fault is corrected.
However, as a result of the fault, the lines of the power distribution system experience over-voltages, which can damage the lines when the fault is not corrected in a timely manner. Thus, fault detection and location estimation is important for the sale and stable operation of ungrounded power distribution systems.
Several methods have been used for locating single-phase-to-ground faults in ungrounded distribution systems. For example, a method described in U.S. Pat. No. 6,721,671 for determining a section of the system having a fault uses a directional element to determine faults on ungrounded power systems, which following enablement under selected input current conditions, determined zero sequence impedance, in response to values of zero sequence voltage and zero sequence current.
Another method described in US 2003/0085715, introduces a measurement signal having a measurement frequency on the line having a fault. The fault location is determined for a selected segment based on a measured residual current corresponding to the measurement signal, and a predetermined relative impedance of the power distribution system. However, usage of additional frequency measurements is not optimal for some applications.
Accordingly, there is a need for determining locations of single-phase-to-ground faults in ungrounded power distribution systems.